Chemical soak to remove furnace contamination without disrupting surface oxide or removing bulk materials

ABSTRACT

An improved method of removing furnace contamination on niobium cavities to increase the quality factor (Q 0 ) and the accelerating gradient (Eacc) of SRF accelerator cavities. Performing a nitric soak at or below 70% concentration removes contamination which can&#39;t be removed by conventional sulfuric/HF EP, HF soaking, which in turn can improve both Q 0  and RF accelerating gradients in niobium. The chemical soak can also remove contamination from a niobium surface without removing the native oxide or bulk niobium removals, such as after infusion or mid-T baking.

This application claims the priority of Provisional U.S. Patent Application Ser. No. 63/187,457 filed May 12, 2021.

The United States Government may have certain rights to this invention under Management and Operating Contract No. DE-AC05-06OR23177 from the Department of Energy.

FIELD OF THE INVENTION

The invention relates to SRF (superconducting radio frequency) technology, and more particularly to a novel method for removing furnace contamination without disrupting surface oxide or removing bulk materials.

BACKGROUND OF THE INVENTION

Conventional furnace contamination removal processes typically use an acid mixture containing hydrofluoric acid (HF) or bi-polar electro-polishing (EP), which methods unfortunately disrupt the surface oxide or remove bulk materials. This surface oxide or bulk removal can negatively affect the fundamental performance of all modern cavity treatments including nitrogen doping, nitrogen infusion, mid-T bakes/oxygen alloying, and conventional bakes.

Accordingly, there is a need for an improved method for removing furnace contamination without disrupting surface oxide or removing bulk materials.

BRIEF SUMMARY OF THE INVENTION

The current invention provides a method of removing furnace contamination on niobium cavities to increase the quality factor (Q₀) and the accelerating gradient (Eacc) of SRF accelerator cavities. Performing a nitric soak, with nitric acid (HNO3) at or below 70% concentration can remove contamination which can't be removed by conventional sulfuric/HF EP, HF soaking, which in turn can improve both Q₀ and RF accelerating gradients in niobium. In addition, the chemical soak can also remove contamination from a niobium surface without removing the native oxide or bulk niobium removals,—such as after infusion or mid-T baking.

OBJECTS AND ADVANTAGES

A first object of the invention is to provide an improved method of removing furnace contamination on niobium cavities to increase the quality factor (Q₀) and the accelerating gradient (Eacc) of SRF accelerator cavities.

A second object of the invention is to provide an improved method of removing furnace contamination without disrupting surface oxide or removing bulk materials.

A further object of the invention is to provide an improved method of removing furnace contamination without roughening the niobium surface like conventional BCP does.

Another object of the invention is to provide a chemical process that may be used after infusion, mid-T bake, and thermal diffusion of native oxide but which does not remove the oxide.

Another object of the invention is to provide a chemical soak that uses a chemical which is readily available and is compatible with almost everyone's chemical systems.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Reference is made herein to the accompanying drawings, which are not necessarily drawn to scale, and wherein.

FIG. 1 is a plot of the quality factor (Q₀) of the cavity alone as a function of the accelerating gradient (Eacc), in MV/m, for a conventional (EP) process versus a nitrogen-doped cavity with furnace contamination and the subsequent contamination removal by a nitric-soak embodiment according to the invention.

FIG. 2 is a plot of the quality factor (Q₀) of the cavity alone as a function of the accelerating gradient (Eacc) for a conventional EP process versus a nitrogen-doped cavity with furnace contamination and the subsequent contamination removal by a nitric-soak percentage embodiment according to the invention.

FIG. 3 is a plot of the quality factor (Q₀) of the cavity alone as a function of the accelerating gradient (Eacc), in MV/m, for a conventional high gradient nitrogen doped cavity process versus a nitrogen-doped cavity with furnace contamination and the subsequent contamination removal by a nitric-soak embodiment according to the invention.

FIG. 4 is a conceptual drawing depicting a nitrogen doping method after heat treatment.

FIG. 5 is a conceptual drawing depicting a nitrogen doping method after electropolishing.

FIG. 6 is a conceptual drawing depicting a nitrogen doping method after a chemical soak.

FIG. 7 is a conceptual drawing depicting a mid-T bake/oxygen alloying method according to the invention after heat treatment.

FIG. 8 is a conceptual drawing depicting the mid-T bake/oxygen alloying method according to the invention after chemical soak.

DETAILED DESCRIPTION OF THE INVENTION

The current invention is a chemical removal step removal method for niobium (Nb) that does not remove the surface oxide or remove any bulk material while advantageously removing furnace contamination. The method can be used on niobium (Nb) cavities to increase the gradient and quality factor (Q₀) of SRF cavities when the niobium metal is heat-treated in the non-reducing environment (vacuum). This method will likely be highly advantageous in the future, where infusion (nanometer nitrogen) and mid-T bake cavity (nanometer oxide) production is opening new avenues for cavity processing. The meaning of the term “mid-T bake” as used herein refers to a bake at 160° C. to 450° C., or essentially raising the oven temperature to 160° C. and higher, but stopping before oxide (NbO5) dissolution occurs. Conventional niobium cavity bakes, which are typically at a range of 100° C. to 160° C., are referred to as “standard bakes” or “magic bakes”. Another conventional bake is at a range of 75° C. to 120° C. The meaning of the term “bake(s)” as used herein refers to a thermal treatment under vacuum (traditionally at a pressure less than ˜5-6 mbar), and usually inside a vacuum oven, or at lower temperatures using the cavity itself as the vacuum vessel while externally heating the cavity.

Various chemical soaks of Niobium (Nb) cavities were tested against control samples as shown in Table I below. FIGS. 1 through 3 present the plotted results of each test condition, with the resultant Q plotted as a function of Eacc(MV/m).

TABLE 1 FIG. Sample ID Test Condition No. SC-11-T109 Baseline (EP ~15° C. + 24 hr bake at 110° C.) FIG. 1 SC-11-T109 3N60 (EP67KC ~13° C.) FIG. 1 SC-11-T109 3N60 + HNO3 70% for 1 hour + HF for 5 hours FIG. 1 SC-14 Baseline-120° C. bake for 24 hours FIG. 2 SC-14 +925° C. anneal followed by 800° C. 3N120 FIG. 2 SC-14 +Nitric soak, 30% for one hour FIG. 2 SC-14 +Nitric soak, 50% for one hour FIG. 2 SC-08 3N60 EP5 FIG. 3 SC-08 +EP50 + 3N60 EP8 FIG. 3 SC-08 +NHO3 70% + HF x 1 FIG. 3

In multiple SRF cavity furnaces, the maximum gradient and Q₀ are limited below expectations, with high variability between furnaces. This is both in high-temperature doped cavities plus light EP and in high-temperature hydrogen degassed plus nitrogen-infused or mid-T baked cavities. The goal is to increase the gradient and Q₀ of SRF cavities when the metal is heat-treated in the non-reducing environment (vacuum).

The current invention is a chemical removal step that does not remove the oxide or remove any bulk material while still removing furnace contamination. This has never been shown to produce any positive result until now. This technique may be highly advantageous in the future, where infusion (nanometer nitrogen) and mid-T bake cavity (nanometer oxide) production is opening new avenues for cavity processing. The meaning of the term “mid-T bake” as used herein refers to a bake at 160° C. to 450° C. or essentially raising the bake temperature to 160° C. and higher but stopping before oxide dissolution occurs. Conventional niobium cavity bakes, which are typically at a range of 100° C. to 160° C., are referred to as “standard bakes” or “magic bakes”. Another conventional bake is at a range of 75° C. to 120° C.

With reference to FIG. 1, there is shown a plot of the quality factor (Q₀) of a single-cell cavity as a function of the accelerating gradient (Eacc) in MV/m for a conventional EP process versus before and after a nitric soak embodiment according to the invention. The conventional EP process (baseline—o) included electropolishing at 15° C. plus a 24 hour bake at 110° C. A nitrogen doping baseline (□) included 3 minutes of nitrogen treatment, followed by 60 minutes of vacuum, followed by electropolishing (EP) removal at 13° C. to remove 67 kilocoulombs (˜5-7 microns) from the surface of the niobium cavity. A nitric soak (▪) post the doping baseline consisted of a nitric soak (nitric acid (HNO3) at 70% concentration) for 1 hour, followed by 5 rinses of HF. After the nitric soak and 10× HF rinse to refresh the oxide, the cavity performed as expected.

Referring to FIG. 2, there is shown a plot of the quality factor (Q₀) of the cavity as a function of the accelerating gradient (Eacc) in MV/m for a conventional EP process versus vs baseline nitrogen doping and two nitric soak embodiments according to the invention. The conventional EP process (baseline—⋄) included a 24 hour bake at 120° C. The nitrogen doping baseline (3N120—o) included 3 minutes of nitrogen treatment followed by 120 minutes of vacuum at 800° C. with a pre-anneal at 925° C. in the same run. Post doping the cavity received a conventional 8-mciron EP done at 13° C. A first nitric soak embodiment (Δ) included a nitric soak (30% concentration) for 1 hour. A second nitric soak embodiment (▪) included a nitric soak (50% concentration) for 1 hour. The 30% nitric soak at 1 hour improved the Q₀, although not as much as expected and the gradient did not improve. The 50% nitric soak improved the Q₀ further to the expected level for 3N120 doping although the gradient did not improve.

With reference to FIG. 3, there is shown a plot of the quality factor (Q₀) of the cavity as a function of the accelerating gradient (Eacc) in MV/m for a conventional high gradient doped cavity vs a reset and redoping in a contaminated furnace followed by a nitric soak A nitrogen doping baseline (□) included 3 minutes of nitrogen treatment, followed by 60 minutes of vacuum, followed by EP of 5 microns. The cavity received a 50 micron reset and a redoping (▪) identical to the baseline doping run with a change to the EP of 8 microns. A nitric soak (●) post the doping reset consisted of a nitric soak (70% concentration) for 1 hour, followed by 5 rinses of HF. The nitric acid returned the high field results up to 32 MV/m.

The chemical removal method of the present invention would be beneficial in the manufacture of SRF cavities or in processing of surface-sensitive refractory metals that requires heat treatment.

Future applications of the technology could be in superconducting niobium accelerators (ILC and EIC) as well as contemporary SRF accelerators, in thin films refractory deposition where bulk or oxide removal is not desirable and which also requires heat treatments, or in furnace-annealed refractory metals that are surface sensitive.

The method is not valid where large bulk removal is allowed, as in bulk BCP/EP of about 20 microns or more.

In multiple SRF cavity furnaces (vacuum ovens), the maximum gradient and Q₀ are limited below expectations, with high variability between furnaces. This is both in high-temperature doped cavities plus light EP and in high-temperature hydrogen degassed plus nitrogen-infused or mid-T baked cavities. The meaning of the term “nitrogen doped” as used herein refers to a vacuum furnace treatment above the native oxide dissolution temperature of approximately 450° C., followed by a gas injection of nitrogen (typically at about 30 mbar) at a higher temperature (traditionally 800-1000° C.) for a short time (20 minutes) and later followed by a vacuum at or below the gas injection temperature, followed by a light EP of 5-20 microns to remove non-superconducting niobium nitride formed in the process.

With reference to FIGS. 4-6, there is shown a representation of the various steps for removing furnace contamination from a nitrogen doped surface after heat treatment and electro polishing . In this conceptual model a furnace contaminate is deposited in or on the nitrogen doped surface while at high temperature during the nitrogen doping treatment (FIG. 4). Post doping there is an EP step (FIG. 5) which removes the nitrides from the surface to reveal the enhanced nitrogen doped niobium surface. In this case the eletro-polishing does not remove the furnace contamination. Only by performing a post EP chemical soak embodiment (FIG. 6) according to the invention is the furnace contamination successfully removed.

Referring to FIGS. 7-8, there is shown the sequence of steps in a mid-T bake/oxygen alloying method according another embodiment of the invention. In this conceptual model a furnace contaminate is deposited on the niobium during the mid-T bake thermal treatment (FIG. 7). Because the surface oxide should not be disturbed, a chemical soak embodiment (FIG. 8) according to the invention is used to remove the contaminate without disturbing the surface oxide. Future applications of the technology could be in superconducting niobium accelerators (ILC and EIC) as well as contemporary SRF accelerators, in thin films refractory deposition where bulk or oxide removal is not desirable and which also requires heat treatments, or in furnace-annealed refractory metals that are surface sensitive.

The method is not valid where large bulk removal is allowed, as in bulk BCP/EP of about 20 microns or more, which would undercut any remaining surface contaminated by removing the metal around the contamination. 

What is claimed is:
 1. A method of removing furnace contamination from the surface of an article of niobium (Nb) to increase the SRF quality factor (Q₀) and the accelerating gradient (Eacc) without removing the native oxide or the niobium, comprising soaking the Nb article in nitric acid (HNO3).
 2. The method of claim 1, comprising the nitric acid (HNO3) is at or below 70% concentration.
 3. A method of removing furnace contamination from the surface of an article of niobium (Nb), comprising: doping the Nb surface with nitrogen; electropolishing (EP) the surface; and soaking the Nb article in nitric acid.
 4. The method of claim 3, comprising the nitrogen doping comprises: applying nitrogen to the Nb article for at least 3 minutes; and applying vacuum for at least 60 minutes.
 5. The method of claim 4, comprising the nitric acid is at or below 70% concentration.
 6. The method of claim 5 comprising the niobium article is soaked at a temperature of at least 160° C.
 7. The method of claim 5 comprising the niobium article is soaked at a temperature of 160° C. to 450° C.
 8. The method of claim 7 comprising the niobium article is soaked while under a vacuum of less than 5 mbar.
 9. The method of claim 7 comprising the niobium article is soaked while under a vacuum of 5-6 mbar.
 10. The method of claim 2 comprising the niobium article is soaked at a temperature of at least 160° C.
 11. The method of claim 2 comprising the niobium article is soaked at a temperature of 160° C. to 450° C.
 12. The method of claim 11 comprising the niobium article is soaked while under a vacuum of less than 5 mbar.
 13. The method of claim 11 comprising the niobium article is soaked while under a vacuum of 5-6 mbar. 